Reconstructing the Alpine crustal architecture of the Ligurian Flysch Nappes through integrated structural cross-sections, RSCM and constrained thermo-kinematic modeling

The structural and thermal architecture of the Ligurian Flysch Nappes in the southwestern Alps remains poorly constrained, despite their key role in late Alpine wedge construction. Reconstructing their burial history and kinematic evolution requires an integrated approach combining structural geometry, independent thermal constraints, and physically consistent numerical modeling. Here, we couple balanced and restored geological cross-sections with Raman Spectroscopy of Carbonaceous Material (RSCM) thermometry to tightly constrain 1D thermo-kinematic modeling of nappe emplacement. Three NE-SW-trending balanced cross-sections (30 to 46.5 km long) were constructed across the para-autochthonous Subalpine foreland, the four main Ligurian Flysch Nappes (Sanremo, Moglio-Testico, Borghetto-Colla Domenica, and Albenga), and the internal Briançonnais domain. Structural restorations provide quantitative constraints on nappe geometries, burial depths, shortening, and kinematics, which are used as boundary conditions for thermal modeling. RSCM thermometry performed on 71 samples yields peak temperatures (T_RSCM) ranging from 140 ± 20 °C to 341 ± 10 °C, systematically increasing with structural depth and toward internal domains. Maximum temperatures are recorded in the inner Subalpine footwall, the deeper nappes (notably the Albenga Nappe), and the Briançonnais units. The thermal overprint in the Subalpine Zone is interpreted as syn-orogenic tectonic burial, supported by the Eocene–Oligocene age of the sampled formations and the thermal continuity observed between nappes and para-autochthonous units. In contrast, a marked thermal inversion in the southwestern frontal sector, where a structural window exposes colder para-autochthonous rocks (~180 °C) beneath the warmer Sanremo Nappe (~250 °C), indicates inherited thermal contrasts during nappe emplacement. These structural and thermal constraints are used to parameterize a 1D thermo-kinematic model of nappe emplacement, in which geometries, thicknesses, and velocities are directly derived from the restored cross-sections. The model accounts for crustal heat production, conductive heat transfer, and basal shear heating. Model results show that the measured T_RSCM values can be fully reproduced by syn-tectonic burial associated with thrust nappe emplacement, assuming a constant geothermal gradient of ~30 °C/km, consistent with independent estimates of paleo-burial depths and eroded overburden. Achieving a satisfactory fit between modeled and measured temperatures requires basal shear heating localized within a finite shear zone, with effective thicknesses ranging from ~4 to 35 m and systematically increasing toward the frontal parts of the orogenic wedge (foreland). The models imply up to ~11 km of eroded overburden in the hinterland and ~6 km in the foreland. Overall, this study demonstrates that structurally and thermally constrained 1D modeling provides a robust and internally consistent framework to quantify nappe emplacement, tectonic burial, and the kinematic architecture of the southwestern Alpine orogenic wedge.